Nonchalking titanium dioxide production



Patented Mar. 2, 1954 NONCHALKING TITANIUM DIOXIDE PRODUCTION Walter R. Whately, Lynchburg, Va., assignor to American Gyanamid Company,

New York,

N. Y., a corporation of Maine No Drawing.

10 Claims.

special properties which render the pigment more suitable for use in the particular vehicle to be employed. In this respect the finishing treatments have included the use of various metal compounds such as zinc oxide, magnesium oxide, and the like materials in a variety of wet and dry classification, grinding, and heat treatments. V

The use of aluminum oxide in titanium dioxide finishing treatments is not new in the art. However, the prior art processes haveutilized finishing treatments wherein the pigment has first been milled and classified in aqueous condition with or without the addition of an aluminum oxide and the pigment was thereafter dried at temperatures not exceeding about 200 C. It has previously been believed that the employment of temperatures in excess of about 200 C. had a deleterious effect upon'the pigment, resulting in discoloration, poor texture, and the like effects.

All of these finishing treatments have been employed without any special regard to the chalking of the pigment either as to its cause or to the actual method of itsprevention. The

industry has found through trial and error that the: addition of certain metallic compounds such that the chalking of titanium dioxide pigments is due, at least in part, to photooxidation. The

titanium dioxide gives off atomic oxygen when exposed to sunlight and thus serves as an oxidant for the vehicles which are employed in the preparation of paints, enamels, and other coating agents. The photooxidation; thus leads to Application May 10, 1950, Serial No. 161,253

the formation of carbon dioxide and produces a gas film between the pigment grains and the binder. Once the pigment has lost its adhesion it sloughs 01f like chalk and the film of paint finally is completely destroyed.

At present there appear to be two methods of preventing or minimizing chalking of titanium dioxide-containing paints. One method consists in coating the pigment particles with a filmof alkali, alkaline earth, or silica which prevents oxygen produced by photooxidation from entering into the pigment-binder interface. The other method consists of addition to the pigment of materials which form surface active groups which decrease the free energy of the titanium dioxide crystal surfaces. It is primarily with this second method of overcoming the deleterious efiects of photooxidation that the present invention is concerned.

I have discovered that hydrous aluminum oxide provides a material whose surface activity apparently almost completely nullifies the free energy of the titanium dioxide crystals when applied to the pigment according to the method of the present invention. Briefly stated, it was discovered that when hydrous aluminum oxide is precipitated upon calcined titanium dioxide from an aqueous solution of an ionizable aluminum compound, followed by heat treating the composite pigment, the chalking resistance of the treated pigment is greatly improved while the other desirable pigment properties do notv suffer appreciable deterioration. I

In its broadest aspect, therefore, the invention comprises the uniform distribution on titanium dioxide pigment of hydrous aluminum oxide followed by a baking treatment at elevated temperatures whereby the hydrous aluminum oxide is at least partially dehydrated and forms a surface active material which minimizes the free energy of the titanium dioxide crystals. The method of .obtaining this uniform distribution of the hydrous aluminum oxide on the pigment consists essentially in admixing the pigment with an aqueous solution of an ionizable aluminum compound and precipitating a hydrous aluminum oxide therefrom. Aluminum compounds which have been found satisfactory in my novel method include aluminum sulfate, aluminum nitrate, aluminum chloride, and the like ionizable aluminum salts as well as various aluminates including sodium aluminate, potassium aluminate, and the like compounds.

It has been found that the use of from about 0.5% to about 5% of hydrous aluminum oxide,

gamer texture and in which poor texture is represented by 1 It will be seen that all of the samples had very good texture characteristics. Also, in the above table the abbreviations mean the following: con.=considerable; sl.=slight; v-. 51.: very slight.

Separate portions of the'above described pigments were exposed in alkyd resin to natural weathering for 21 months. The results are tabu- Calcined anatase T102 was hydroclassifled to separate out particles larger than 4 microns in diameter. 1200 g. of this T102 was slurried in water to obtain a mixture containing 241 g./liter of TiOz. Solutions of titanium sulfate, sodium silicate and aluminum sulfate were added to the slurry in this order. The quantities of the addition agents were sumcient to provide 1 part of TiO2, 1 part of SiOz and 2 parts of A1203 for each 100 parts of TiOz in the slurry. The mixture was heated to 70 C. after which sodium carbonate solution was added until the pH of the slurry was 5.0, after which caustic soda solution was added to obtain a pH of 7.5. v

The treated slurry was then filtered, washed and dried at 130 C; to a moisture content of 0.1%. The dried, treated pigment was divided into 3 parts. One sample was kept as a control without further treatment, while the remaining two samples were heated for minutes in an electric muflle furnace at temperatures of 500 C. and 700 C. respectively. The dehydrated samples were thereafter milled to lower the oil absorption properties. The samples were ground in linseed oil, formulated at 23% pigment volume concentration and exposed in an accelerated weathering unit both as untinted and as gray tinted paints. The results are tabulated below.

Table III Ohalking Dehydration Sample No. Temper- Gray Tinted ature, Untinted, C. 100 hrs.

60 hrs. 80 hrs.

130 v. sl. :1. 600 110%.... trace 700 trace v. sl

EXAMPLE 3 weathering machine as untinted and gra'y'tmted paints in the'form 'of 23% pigment volume concentration in linseed oilpaint. The chalking results are tabulated'bellow.

' Table IV Dehydration Chnlking-Untinted Paint Temperature, Q 626 hrs. sea hrs. 720 hrs. 745 hrs. 767 hrs.

sl sl bad had. 550 v.sl v. sl.-- v. sl 81.

Table V Chalking-Gray Tinted Paint Dehydration Temperature C.

508 hrs. 550 hrs. 608 hrs. 667 hrs.

130 v. v. sl v. s] sl bad. 550 trace v. v. 51.... v. 81..... con.

EXAMPLE 4 To 5 liters of an aqueous slurry of hydroclassified calcined rutile Ti02 containing 1000 g. of pigment was added a solution of aluminum sulfate containing the equivalent of 2.0% A1203 based on the weight of TiO2. The slurry was heated at 70 C. for one hour after which sodium carbonate solution was added until a pH of 5.0 was reached and then NaOI-I solution was added to obtain a pH of 7.8. The slurry was filtered and washed until the soluble salt content was less than 0.1%. Thereafter the filter cake was dried at 130 C. The dried material was then heated to 800 C. in an electric muflle furnace and held at this temperature for 10 minutes. After cooling the pigment was jet milled. The treated pigment had excellent resistance to chalking when incorporated in an alkyl enamel formulation and exposed to natural weathering for a period of several months.

EXAMPLE 5 Calcined rutile TiOz was treated according to the procedure described in Example 4 except that the amount of aluminum nitrate employed contained the equivalent of 5.0% A1203 based on the weight of the pigment. The chalk resistance of the product was substantially the same as that of the product of Example 4.

EXANIPLE 7 A 5 liter slurry of calcined rutile T102 was treated according to the procedure set forth in Example 4 except that a titanium sulfate solution containing the equivalent of 1.0% T102, based on the weight of the pigment, was substituted for the aluminum sulfate solution. The product had poor chalk resistance when tested in the same manner as the product of Example 4.

EXAMPLE 8 The procedure of Example '7 was repeated except that solutions of titanium sulfate and sodium silicate containing the equivalent of 1.0% T102 and 1 S102, based on the weight of the pigment,;we re used in place of the aluminum sulfate solution. The product had poor chalk resist nQ -"HQQLJ; I

To 2.66 liters of an aqueous slurry of hydroclassified rutile TlOz containing 500 g. of H was added an aqueous solution of sodium aluminate equivalent to 1.5% A1203 based on the TiOz present. The slurry was heated to 70 C. and held at this temperature for one hour after which dilute H2804 was added until a pH of 7.0 was obtained The slurry was thereafter agitated for one hour at70 C- after whichv it was filtered and the filter cake was washed with 5 liters of water. The filter cake was then dried at.135? C. to a moisture contentof less than 0.2%. The dried material was heatedtoBOO? G. in an electirc furnace and held at this temperature for minutes. After cooling the bakedpigmentrwas jet milled.

The product obtained had excellent resistance to chalking when tested in an accelerated weathering testing machine.v

" EXANIPLE 10 The procedure of Example 9 was repeated'except that an equivalent amount of potassium aluminate wa 'employedinstead of sodium aluminate. The product obtained had chalk resistance equal to that of the product of Example 9.

EXAMPLE 11 1. In a process of preparing a titanium dioxide pigment possessing improved resistance to chalking wherein the particles of said pigment in aqueous slurry are coated with a hydrous aluminum oxide, the weight of said oxide as A1203 being about 0.5% to about 5% of the weight of said pigment, andlsaidcoated particles are then heated, the step of heatingcthe thus coated pigment between acout 500 C. and the higher temperature atwhich the pigment particles increase in size, until. the hydrous; aluminum oxide is at least partially dehydrated.

8 2. Process according to claim 1, wherein the temperature. to which the coated pigment is heated is between 500" and 800 C. I 3,- P.rocess' according to claim .2, whereinthe coated pigment isheated to between 500 and 800 C. for about lflminutes. 4. Process according to claim 3, wherein the weightof the hydratedaluminum oxide as A1203 on the titanium dioxide particles is 1-2% of the weight of said particles. 5. In a process of preparing a titanium dioxide pigment wherein a slurry of a titanium dioxide pigment is slurried with an aqueous solution of an ionizable aluminum containing salt, and the aluminum content of said salt is precipitated on thefparticles of said pigment as a hydrous alumium oxide, the weight of said oxide as A1203 being 0.5% to 5% of the weight of said titanium dioxide, andthe thus coated pigment is filtered from said slurry, thestep of heating the coated pigment between about 500 C. and the higher temperature at which the pigment particles increase in size until the hydrous aluminum oxide is at least partially dehydrated.

6.- Process according to claim- 5 wherein the temperature to which the pigment is heated is between 500 and 800 C.

'2. Process according to claim 6 wherein the coated pigment is heated to between 500 and BMC. for about 10 minutes.

8. Process according to claim '7 wherein the weight of the hydrated aluminum oxide, as A1203, on the titanium dioxide particles is 1-2% of the weight of said particles.

9. Process according to-claim 5 wherein the ionizable aluminum containing compound is aluminum sulfate.

10; Process according to claim 5 wherein the ion aable aluminum containing compound is sodium aluminate.

WALTER R. l/VHATELY.

References Cited inthe file of this patent UNITED STATES PATENTS Number Name Date r 2,284,772 Seidel June 2, 1942 2,346,322 Nelson Apr. 11, 1944 2,369,246- Peterson Feb, 13, 1945 2,441,225 Pechukas May 11, 1948 2,559,638 Krchma et al.- July 10, 1951 

1. IN A PROCESS OF PREPARING A TITANIUM DIOXIDE PIGMENT POSSESSING IMPROVED RESISTANCE TO CHALKING WHEREIN THE PARTICLES OF SAID PIGMENT IN AQUEOUS SLURRY ARE COATED WITH A HYDROUS ALUMINUM OXIDE, THE WEIGHT OF SAID OXIDE AS AL2O3, BEING ABOUT 0.5% TO ABOUT 5% OF THE WEIGHT OF SAID PIGMENT, AND SAID COATED PARTICLES ARE THEN HEATED, THE STEP OF HEATING THE THUS COATED PIGMENT BETWEEN ABOUT 500* C. AND THE HIGHER TEMPERATURE AT WHICH THE PIGMENT PARTICLES INCREASE IN SIZE, UNTIL THE HYDROUS ALUMINUM OXIDE IS AT LEAST PARTIALLY DEHYDRATED. 